A painting by the Belgian surrealist artist René Magritte was stolen from a Brussels museum today in a daring daylight raid by two armed men who rang the doorbell before putting a gun to the concierge's head.

The 1948 painting, Olympia, a nude portrait of the artist's wife Georgette, is said to be worth up to €3m (£2.75m). It was hanging in Magritte's former terrace house, which is open as a museum by appointment only.

The museum had been open for 10 minutes when a man rang the doorbell asking if visiting hours had started. He put a revolver to the museum attendant's temple and allowed his accomplice inside. The two men, who were not masked, rounded up the museum staff and visitors – a Japanese couple – and made them kneel in the courtyard. No one was hurt and the pair left carrying the 60cm by 80cm painting and got into a car.

As soon as they had smashed the glass plate protecting the work, an alarm sounded but by the time the police arrived, the thieves, who spoke English and French, had made their getaway.

Magritte lived with his wife for more than 20 years in the house in the northern Brussels suburb, and painted some of his most famous works there. The house also became a meeting point for other surrealists in the Belgian capital.

Now a small museum, it is distinct from the big new Magritte museum which opened in Brussels in June. Since the opening of the vast new Brussels museum, there has been a surge of interest in Magritte's witty and surrealist images, such as his famous painting of a pipe with its warning: "This is not a pipe".

The stolen painting, posed by Magritte's wife, shows a woman reclining with a shell balanced on her stomach.

Maja Pertot Bernard of the Art Loss Register said Magritte was not a painter whose works were often stolen and there were very few missing Magrittes.

"This painting, which is highly recognisable, is very unlikely to be attempted to be sold on the open market," she said. "In thefts like these, the paintings either tend to turn up very quickly when the thieves realise it's a lost cause, or if they do go missing for a long time, they often change hands so many times that the final seller doesn't realise there is a problem with the painting."

She said paintings were sometimes held for ransom. "More often, they are used as collateral, in exchange for something, or to pay off a debt."